There are a number of different commands you can use for switching to a buffer by its number. For example, if you want to switch to the buffer assigned 7, you can do :7b or :e #7. However, there is a quicker way than typing out either of those commands.

You may already be familiar with CTRL-^ for switching to the alternate file which is generally the previous buffer. If you precede that command with a number, you will switch to the buffer with that number instead of the alternate file.

From normal mode

7 Ctrl-^

will switch to buffer 7.

See :h Ctrl-^ for more details.